A metal-oxide-semiconductor field-effect transistor (MOSFET), or simply a field-effect transistor (FET) or a transistor, is widely used in integrated circuits (ICs), including digital integrated circuits, radio frequency (RF) analog circuits, etc. The gate length of a transistor is reduced or scaled down to increase the packing density of transistors in an IC and to increase the speed performance thereof. However, transistors with aggressively miniaturized gate lengths suffer from undesirable short-channel effects, such as increased off-state leakage current.
To effectively suppress short-channel effects in transistors having gate lengths smaller than about 20 nanometers (nm), one way is to employ a semiconducting channel with reduced thickness, e.g. less than 5 nm. A transistor having a semiconductor channel with a thickness less than one-third or even one-quarter of its gate length is commonly known as an ultra-thin body transistor. Ultra-thin body transistors may employ ultra-thin semiconductor channel materials. One type of ultra-thin semiconductor materials with high mobility is black phosphorus (BP). BP is a layered material. A monolayer of BP, termed “phosphorene,” is a semiconductor with a direct band gap of about 2 eV at the Γ point of the first Brillouin zone. When a number of layers of phosphorene are stacked, the multi-layer BP has a reduced band gap, with the band gap reduced to about 0.3 eV for bulk BP.
However, existing methods of fabricating transistors with BP channel suffer from various problems. One problem is that many of these transistors were realized using mechanically exfoliated BP layers. It is difficult to employ the mechanical exfoliation technique to mass produce transistors with BP channels for large-scale ICs. Another problem is the exposure of the BP layers to air or moisture in the ambient prior to gate dielectric formation. Upon exposure to air, the surface of a BP layer becomes oxidized, and adsorbs moisture from the air. This irreversibly converts the BP surface to phosphorus oxide (POx) compounds, resulting in non-uniform degradation of the surface and/or edge properties of the BP layer.